I miss her.
After months of frequenting the road on which Donna's house was situated, the Doctor began to get involved in her life, still without her knowing of it, through things as small as boxes of chocolates through the door, to larger things like job opportunities, and once he even managed to anonymously mend Wilf's telescope.
Eventually, he cracked, and had to see her again, but he still couldn't brave her house, in case he met Sylvia Noble. Rose did not understand this phobia. Other than not owning a webcam, she had seemed fine for the brief time Rose had met her, but she supposed having a girl with a ginormous gun in her house at the same time as a fleet of invading Daleks that moved the Earth to a different part of space so they could destroy the universe might have helped to subdue her somewhat.
The Doctor was contemplating how to meet Donna somewhere other than her house when he bumped into her again – literally – in a coffee shop.
"Oi, watch it!" she said, as he nearly knocked her coffee over her. "Oh, it's you – Doctor Frankenstein," and he was pleased to see her smile slightly.
"Just 'the Doctor'," he told her.
"Right. But seriously, you need a better nickname. How long have people called you the Doctor?"
"A while," he replied, cryptically. "So how's your arm?"
"Fine, the cast is off and everything."
"And how did your mother take the loss of her car?"
"Not well," Donna muttered darkly, and he needed no elaboration to imagine it. "Anyway, gotta go, but we should see each other again, yeah?" She was already backing towards the exit. "Bye, stick insect."
What was it with Donnas and nicknames? Spaceman, skinny boy, now stick insect?
Somehow, that was all it took for friendship to start. It was like they needed each other after being so closely linked by the metacrisis. They saw each other again soon after, mainly to do with the fact that the Doctor began to haunt the coffee shop in the hope of meeting her, and they got into the habit of regularly meeting up. She came to the Doctor and Rose's countless times, the Doctor and Donna bickering for much of the time, and at last, after trying so hard to put it off, the day came when the Doctor visited her house.
"Why are the mothers always so scary?" he complained to Rose afterwards. "I mean, yours kissed me, Martha's slapped me, and Donna's... well, she's terrifying."